Function which returns type "String" even if "returnType" is "boolean"

We use a function for validation of values which we call from an outbound resource mapping. The function has "returnType" set to  <returnType>t:boolean</returnType> yet logging the value in the outbound mapping script expression returns "String":

log.info("#### AD RESOURCE ###: class : {}", attributeValidation.execute("nameValidation", mapFixedFirst).getClass().toString())

Returns

INFO (com.evolveum.midpoint.expression): #### AD RESOURCE ###: class : class java.lang.String

In general the output of the function acts like a string.

Attaching the function in function library.